1. Field of the Invention
The present invention relates to a positive electrode mixture paste, a positive electrode, a nonaqueous electrolyte secondary battery, and a manufacturing method of a nonaqueous electrolyte secondary battery.
2. Description of Related Art
Conventionally, there has been known a nonaqueous electrolyte secondary battery including a positive-electrode active material (e.g., a lithium nickel manganese oxide having a spinel structure) having an operation upper limit potential of 4.35 V or more on the metal lithium basis. In the nonaqueous electrolyte secondary battery, when a potential of the positive-electrode active material (equivalent to a potential of a positive electrode) reaches 4.35 V or more by performing initial charge or the like, a solvent in a nonaqueous electrolyte is decomposed by oxidation on a surface of the positive-electrode active material, so that hydrogen ions generated hereby are reacted with fluorine ions in the nonaqueous electrolyte, which might generate hydrofluoric acid (HF). Due to an action of the hydrofluoric acid, transition metals in the positive-electrode active material are eluted, which might decrease battery performance.
In this regard, Japanese Patent Application Publication No. 2014-103098 (JP 2014-103098 A) describes a technique in which lithium phosphate is contained in a positive electrode mixture layer, so that hydrofluoric acid generated as described above is reacted with lithium phosphate (Li3PO4) so that the hydrofluoric acid is reduced, thereby reducing elution of transition metals in a positive-electrode active material. More specifically, in JP 2014-103098 A, the positive-electrode active material, a conductive material, a binder, and lithium phosphate, and a solvent are kneaded (stirred), so as to manufacture a positive electrode mixture paste. Then, the positive electrode mixture paste thus manufactured is applied to a power collection member and then dried, so as to manufacture a positive electrode including a positive electrode mixture layer containing lithium phosphate.
However, in the manufacturing method described in JP 2014-103098 A, a good dispersion degree of lithium phosphate in the positive electrode mixture layer might not be obtained. More specifically, when the positive electrode mixture paste is manufactured, for example, lithium phosphate is aggregated (or aggregated lithium phosphate cannot be separated), which decreases dispersibility of lithium phosphate in the positive electrode mixture paste, thereby resulting in that a dispersion degree of lithium phosphate in the positive electrode mixture layer might be lowered. Note that, if a kneading (stirring) energy of the positive electrode mixture paste is raised in order to raise dispersibility of lithium phosphate, the positive-electrode active material is broken. For this reason, any method that increases a kneading (stirring) energy so as to raise the dispersibility of lithium phosphate cannot be employed.
If a dispersion degree of lithium phosphate in the positive electrode mixture layer is not good (lithium phosphate is not uniformly dispersed in the positive electrode mixture layer to some extent), lithium phosphate cannot be appropriately reacted with hydrofluoric acid generated on surfaces of many positive-electrode active materials dispersed in the whole positive electrode mixture layer, which might not be able to prevent elution of transition metals in the positive-electrode active material appropriately.